The present invention relates to a device for monitoring the particle contamination in flowing hydraulic fluids according to the preamble of claim 1 and a method for monitoring the particle contamination according to claim 7.
The quality of hydraulic fluids is essential for ensuring the proper operation of safety-critical subsystems in aircrafts such as flaps, slats, landing gears, etc. Contamination of the hydraulic fluid can severely damage mechanical components in the hydraulic system. Therefore, a stringent contamination control is required at all levels of maintenance to ensure flight safety and the highest degree of hydraulic system readiness.
Contamination present in an operating hydraulic system is normally originated at several different sources. Maintenance malpractices introduce large amounts of external contaminates. Wear and chemical reactions are also factors in hydraulic system contamination. The types of contamination are generally classified as organic, metallic solids, non-metallic solids, foreign fluids, air and water.
Organic solids are produced by wear, oxidation or polymerization. Minute particles of O-rings, seals, gaskets and hoses are present due to wear or chemical reactions.
Metallic contaminates are almost always present in hydraulic systems. These particles are the result of wear and scoring of bare metal parts and plating materials such as silver and chromium.
Inorganic or nonmetallic solids include dust, paint particles, dirt and silicates. These particles are often drawn into a hydraulic system from external sources. Disconnected hoses, lines and components are entry points. The wet surface of a hydraulic piston shaft may also draw some contaminates past the wiper seals and into the hydraulic system.
Air contamination, dissolved, entrained or free floating can adversely affect a hydraulic system. Air entrained hydraulic fluid can cause severe mechanical damage from pump cavitations, system pressure loss or slow or erratic flight control movements.
Water is a serious contaminant of hydraulic systems. Dissolved, emulsified or free water may result in the formation of ice or oxidation or corrosion products of metallic surfaces.
Foreign fluids are generally the result of lube oil, engine fuel or incorrect hydraulic fluid having been inadvertently introduced into the hydraulic system during servicing. The effect of foreign fluids other than water depends on the nature of the contaminant. Compatibility of materials of construction, reactions with hydraulic fluid and water, and changes in flammability and viscosity are all affected. The effects of such contamination may be mild or severe depending on the contaminant, amount (quantity) and length of time it has been in the system.
Contamination in oil is specified from particle count. Two basic methods are used: Laser based particle count analysis equipment gives directly information on particle sizes (micron=μ) and figures within specified size ranges. The other method utilize filtering an oil sample through a very fine mesh filter paper. The particles on the surface of the filter paper are then monitored in a microscope and compared to standard contamination pictures to indicate the degree of contamination.
Instead of specifying particle counts contamination is separated into classes defined in two major systems ISO (International Standard Organization) and NAS (National Airspace Standard). Each class defines a range of counts within an exponential scale. Unfortunately, the two systems are not identical and can not be converted in simple mathematics. However, some simple guidelines can be given. First of all let's look at the two systems.
The NAS system divides particles in 5 ranges. Furthermore, the NAS system specifies different counts within each particle range to score a specific class. In practice, oil samples will show up to gain almost the same NAS class rating within the different particle ranges. The system is designed to match the most common found contamination which has really many small particles and fewer big particles. A typical oil analysis can for example have counts divided in the 5 classes. The resulting NAS class according to NAS1638 is defined as the particle count with the highest (worse) score, and only this class is specified.
Currently the quality of hydraulic fluids is generally monitored off-line. During A/C service, fluid samples are tapped from the hydraulic system and their quality is assessed with laboratory equipment off-line. This procedure is time-consuming and costly. Necessary service actions are taken according to fixed schedules rather than on demand.
U.S. Pat. No. 4,323,843 discloses an apparatus for detecting ferrous contamination in a fluid such as engine transmission lubricant. The apparatus has two spaced apart electrodes, a magnetic flux extending between the electrodes, the lines of force of which are substantially rectilinear, the electrodes being connectable to a circuit for signaling when an electrically conductive path is formed from one electrode to the other by metal particles attracted to the flux and formed into an electrode spanning bridge. For preference the sensor is a hollow plug, which in use is screwed into a transmission housing port, and has a flat end wall in contact with the lubricant in the housing. The plug contains a magnet having both poles disposed towards the plug end wall, a portion of the end wall acting as one electrode. A ferromagnetic disc mounted externally from the plug end wall, and electrically insulated therefrom, overlies the poles. The disc acts as a second electrode and serves to direct the magnetic flux rectilinearly across the gap between the electrodes. The electrodes are connectable to circuit means whereby a change in interelectrode resistance may be detected. A major drawback of this detection device is the fact that only ferrous contamination can be detected. Therefore the effectiveness is very limited with respect to the efforts made and the costs involved.
U.S. Pat. No. 5,754,055 describes a lubricating/hydraulic fluid condition monitor in which a coaxial microwave resonator is placed in a fluid conduit to determine changes in the chemical properties and debris concentration is disclosed. Microwave radiation is applied to the resonator for measuring the resonant frequency and resonator Q. An externally powered electric or magnetic field is used to alternately align and misalign debris in the fluid while the resonator properties are being measured. A logic unit automatically generates tables of resonant frequency and Q versus resonator mode and external field strength. This set of tables constitutes a fingerprint of the fluid condition. By matching the fingerprint against a set of fingerprints taken under known conditions, the condition of the fluid is determined. Changes in the fluid's dielectric constant caused by oxidation or the presence of water, changes in the concentration and size of conducting particles from bearing wear, and changes in viscosity all affect the fingerprint; and thus, can be monitored in real time. In a variation of the invention, a lumped-circuit resonator printed on a microwave circuit board is used as the sensor. In a further variation, a transmission-line resonator printed on a microwave circuit board is used as a sensor. In yet another variation the resonator is a lumped circuit wave guide structure through which the fluid flows. In still another variation, time domain reflectometry is used in a transmission line having one end immersed in the fluid. A major drawback of this solution is the use of microwave frequencies which cause problems for in-flight operation. Especially in the vicinity of fly by wire systems this detection device can not be used during operation of an aircraft.
U.S. Pat. No. 4,013,953 discloses an optical oil monitor that measures particle contamination in oil by passing light through an oil sample and picking up the light that is scattered at 90° by the particle contamination and measures chemical breakdown by the attenuation of the light passing substantially straight through the oil with a second photo sensor. Alternately a sample and a reference are passed between the light responsive sensors for error correction and calibration so that each sensor will have an output signal alternating between a sample signal and a reference signal. The sample and reference are housed within a rotor provided with vanes so that it may be driven as a pump by a motor or be driven by fluid flow as a turbine. When the rotor acts as a turbine, the frequency of the light responsive sensors will be correlated to the fluid flow rate indicated by the turbine turns so that an appropriate frequency responsive gauge is provided in circuit to monitor the fluid flow. The peak signals of a peak detector indicating the particle count are summed up to provide an output corresponding to the amount of contamination. Major drawbacks of this device are the complex mechanical construction including a turbine sample rotor in the lubrication oil flow as the main constructive part. This may lead to mechanical defects and failure of the entire hydraulic system. Further, no indication of the particle size can be provided by the known solution. Finally this known solution is costly and uses relatively large space.